1. Field of the Invention
The present invention is related to calibrating spatial light modulators having reflective pixels, and more particularly to calibrating reflective spatial light modulators utilized in maskless lithography systems.
2. Background Art
Spatial light modulators (SLMs) (e.g., digital mirror devices (DMDs), liquid crystal displays (LCDs), grating light valves (GLVs), or the like) are used in many different environments to pattern objects or project patterns, for example, lithography (e.g., maskless lithography), televisions, biomedical systems, biotechnology systems, etc. SLMs can include an active area having an n×m (wherein n and m are integers equal to or greater than 1) array of active devices (or pixels) (e.g., an array of mirrors on the DMD or GLV or an array of reflective/transmissive devices on the LCD). Each active device is individually controlled to move (e.g., tilt, rotate, pivot, etc.) the active devices between ON and OFF through one or more discrete states. For example, if the active devices are mirrors on the DMD or GLV, each of the mirrors is individually controlled to rotate or bend between binary or multiple positions.
FIGS. 1, 2, and 3 show conventional systems 100, 200, and 300, respectively, for illuminating an SLM array, so that patterned light is formed and directed from the SLM array towards a substrate. As is known, the illumination optics and the SLM optics can include one or more optical elements (e.g., lenses, mirrors, etc.).
As discussed above, one application for an SLM, or an array thereof, is in maskless lithography. Lithography is a process used to create features on the surface of a substrate. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer or glass substrate. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image (e.g., a pattern) formed by the SLM, or array thereof. The image is projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, an eximer laser, x-ray, ion, electron, or photon lithography can each require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer (e.g., photoresist) deposited on the surface of the wafer. These changes correspond to features in the image projected onto the wafer during exposure. Subsequent to exposure, the layer can be processed to produce a patterned layer. The pattern corresponds to the features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time with the image formed by the SLM, individual fields are scanned onto the wafer one at a time. This is accomplished by moving the wafer and controlling active devices on the SLM, such that the imaging slot is moved across the field during the scan. The wafer stage must then be stepped between field exposures to allow multiple copies of the pattern formed by the active devices on the SLM to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.
In order to ensure patterning is accurate, control of the active devices must be accurate. The accuracy is based on calibrating the active devices. One method uses individual calibration of the SLM pixels, which requires the capability to measure the tilt of each pixel resulting from a certain voltage applied to the pixel. However, viewing one pixel at a time by turning off all other pixels is not desirable because the calibration time becomes prohibitive.
Another calibration method is based on imaging all pixels of the SLM array on a CCD array using projection optics of the lithography tool to resolve each pixel. The light reflected from each pixel is then mostly concentrated within the bounds of the pixel's geometrical image. The problem of such an approach is that the image of the tilting micro-mirror pixel produced by the projection optics resolving this pixel does not depend (or depends very weakly) on the pixel mirror tilt. The modulation of the tilting mirror pixel image is based on the fact that the tilting mirror pixel deflects the portion of light that falls on it away from the projection optics entrance. This modulation therefore depends on the object size numerical aperture (NA) of the projection optics. The imaging projection optics, used to project the image of the SLM onto the substrate, samples the fraction of the zero order diffraction lobe from the pixel (thus the pixel is sub-resolution when it is imaged during printing). The calibration projection optics resolving the pixel has to capture many diffraction lobes from the pixel. Therefore, tilting the pixel within the range of the tilt angles of interest would not result in a significant modulation of the resolved image of the pixel. Resolution of the individual tilting mirror pixel and the (strong) modulation of the resolved image depending on the tilt are both required for the calibration, but these properties are mutually exclusive, as explained above.
Therefore, what is needed is a system and method that allows for calibration of all individual pixels in an SLM substantially simultaneously, while still allowing for images of the pixels to be well resolved and to allow each image to strongly modulate with movement (e.g., tilting, pivoting, rotating, etc.) of the pixel. In this way the image of the pixels depends strongly on the movement of the pixels.